Optical rings: a large number of gravitational lenses?

We propose that a large number of gravitationally lensed rings should be observable in the optical. We argue that the probability of forming a ring is likely to be similar for optical and radio sources. The number of optical rings in the sky can then be estimated from the rate of occurrence of rings in radio sources, and the observed number of optical sources. This gives ∼100 optical rings per square degree, or a few million over the whole sky, to a magnitude limit of B=26. Most of the rings should be substantially bluer than the foreground lenses, allowing them to be detected with multicolour imaging